ATM originated as a telecommunication concept defined by the Comite Consulatif International Telegraphique et Telephonique (CCTI), now known as the International Telecommunications Union (ITU), and the American National Standards Institute (ANSI) for carrying user traffic on any User to Network Interface (UNI) and to facilitate multimedia networking between high speed devices at multi-megabit data rates. ATM is a method for transferring network traffic, including voice, video and data, at high speed. Using this connection oriented switched networking technology centered around a switch, a great number of virtual connections can be supported by multiple applications through the same physical connection. The switching technology enables bandwidth to be dedicated for each application, overcoming the problems that exist in a shared media networking technology, like Ethernet, Token Ring and Fiber Distributed Data Interface (FDDI). ATM allows different types of physical layer technology to share the same higher layer--the ATM layer.
ATM uses very short, fixed length packets called cells. The first five bytes, called the header, of each cell contain the information necessary to deliver the cell to its destination. The cell header also provides the network with the ability to implement congestion control and traffic management mechanisms. The fixed length cells offer smaller and more predictable switching delays as cell switching is less complex than variable length packet switching and can be accomplished in hardware for many cells in parallel. The cell format also allows for multiprotocol transmissions. Since ATM is protocol transparent, the various protocols can be transported at the same time. With ATM, phone, fax, video, data and other information can be transported simultaneously.
The components of the ATM header consist of the following fields. A generic flow control (GFC) field provides flow control; a virtual path identifier (VPI)/virtual channel identifier (VCI) field allows the network to associate a given cell with a given connection; a payload type identifier (PTI) field indicates whether the cell contains user information or management related data and is also used to indicate a network congestion state or for resource management (i.e., the EFCI bit which is part of the PTI field); a cell loss priority (CLP) field indicates that cells with this bit set should be discarded before cells with the CLP bit clear; a header error check (HEC) field is used by the physical layer for detection and correction of bit errors in the cell header and is used for cell delineation.
The provisioning of an ATM network connection may include the specification of a particular class of service. The following list the various classes of service currently defined in ATM. Constant bit rate (CBR) defines a constant cell rate and is used for emulating circuit switching (e.g., telephone, video conferencing, television, etc.). Variable bit rate (VBR) allows cells to be sent at a variable bit rate. Real-time VBR can be used for interactive compressed video and non real-time can be used for multimedia e-mail.
Available bit rate (ABR) is designed for data traffic (e.g., file transfer traffic, etc.) and is the class service connected with resource management. The source is required to control its rate depending on the congestion state of the network. The users are allowed to declare a minimum cell rate, which is guaranteed to the virtual circuit by the network. ABR traffic responds to congestion feedback from the network.
A fourth class of service, unspecified bit rate (UBR), is utilized by data applications that are not sensitive to cell loss or delay and want to use leftover capacity. During congestion, the cells are lost but the sources are not expected to reduce their cell rate.
ATM is a connection oriented transport service. To access the ATM network, a station requests a virtual circuit between itself and other end stations, using the signaling protocol to the ATM switch. ATM provides the User Network Interface (UNI) which is typically used to interconnect an ATM user with an ATM switch that is managed as part of the same network.
Currently, more and more high speed networking applications are utilizing ATM technology. Some of these network applications require very high reliability connectivity between network elements. This connectivity reliability also has a time critical element in that in the event of a connection failure, the recovery mechanism must operate very quickly.
Such connectivity reliability is especially required within a local area network (LAN) environment that utilizes fast communication paths coupled with short response times, as opposed to a wide area network (WAN) environment.
As ATM switches and edge devices are used more frequently today to construct LAN and WAN connectivity via point to point connections (as opposed to bus connections such as Ethernet, Token Ring, FDDI, etc.) it is becoming more and more crucial that ATM entities also support such connectivity reliability.